Alternating-current dynamoelectric machine



y 1939- E. w. KREBS 2 ,160,594

ALTERNAT ING-CURRENT DYNAMO-ELECTRIC MACH INE Original Filed April 1,1936 2 Sheets-Sheet 1 Fig.1.

Fig. 3.

Inventor: Ernst W. Kr bs,

His Attorney.

y 1939- E. w. KREBS 2,160,594

ALTERNATING-CURRENT DYNAMO-ELECTRIC MACHINE Original Filed April 1936 2Sheets-Sheet 2 Invent or: EPFW st W. Krebs,

60%] EJMZL b9 l-lis Attorney.

UNITED STATES PATENT-1 OFFICE 2,180.5 ALTERNATING-CURIENT DYNAMO-ELECTRIC moms r Ernst W. Krebs, Berlin-Beinickendori, Germany, assignorto General Electric Company, a 'corporation or New York Originalapplication April 1, 1936, Serial No.

Divided and this application November 22, 1938, Serial No. 241,849. InGermany April 1., 1935 12.01am (Cl. 171-223 This application is adivision 01' my copending application Serial No. 72,196, flied April 1,1936, and assigned to the same assignee as my present application.

My invention relates to alternating-current dynamo-electric machines andhas for its principal object the utilization of self-excitation oi suchmachines for control or regulation purposes.

--It is an object of my invention to provide a precise, thoroughlyreliable system for regulating. limiting, or controlling variablequantities, such as speed, pressure, temperature, voltag current, etc.

- It is a further object of my invention to provide such a system inwhich the operation is full automatic.

Still another object of my invention is to provide a regulatingsystemwhich is entirelyelectrical, avoiding the eilects of friction, inertia,

, centrifugal force, and the like.

Other and further objects and advantages will become-apparent as thedescription proceeds.

It is known that self-excitation takes place in rotatingalternating-current machines under certain conditions. Thus, excitationor magnetiza tion of the field may be brought aboutin consequence ofcapacity eflects, for example, by connecting condensers to the machine.Or, in cases of commutator machines, the magnetization may also beproduced'by armature windings.

Such self-excitation takes place in both synchronous and asynchronousmachines regardless of whether the machines are designed for singlephaseor po yphase service. 5 In accordance with my invention in its preferredform, an alternating-current generator is driven from a shaft. the speedof which is to be regulated, or which rotates at a speed responsive tosome other quantity to be regulated, such as fre- 40 quency, voltage,pressure, etc. The appearance at voltage at the terminals or thegenerator is made criticalto a predetermined speed by'utilizationoi thecapacity eflect, andthe generator out-' put is eitherdlrectly orindirectly utilized for 43 operating an alarm or for operating devicescorrecting the speed of the apparatus driving the generator. Y

The invention will be understood more readily from the followingdetailed description when conso sidered" in connection with theaccompanying drawings," and those features of 'theinvention which arebelieved tobe novel and'patentable will be pointed out 'in the claimsappended hereto. In the drawings, Fig. 1 represents 66 schematically asynchronous machine arranged in accordance with an embodiment of myinvention; Fig. 2 is a graph explanatory or the principle of operationof the apparatus of Fig. 1 at a predetermined speed; Fig. 3 is a graphexplanatory of the operation at varying speeds; 5 Fig. 4 is a circuitdiagram representing schematically another embodiment of my inventionemploying an asynchronous machine; Fig. 5

is a circuit diagram of a unitary frequency-responsive machineconstituting another embodi- 10 ment of my invention;' Fig. 6 is aschematic diagram of a'turbine. governor embodying my invention; andFig. '7 is a schematic diagram representing the stator core ortheembodiment of Fig. 5. 16

Referring more in detail to the drawings, a three-phase synchronousgenerator II is shown in Fig. l with a stator winding l2 connected tothe condensers is arranged in Y or star connection. The rotor I4 is ofthe salient pole type and, 20 for the sake of simplicity, carries nowinding. The rotor I4 is mounted on a shaft I 5 and the apparatus isdesigned to be responsive to .the speed of rotation of the shaft l5,which'is driven by a suitable source of mechanical power. Suit- 5 ablemeans are provided for obtaining a response to the voltage induced inthe stator windingsl2.

By way of example, I have shown a voltmeter I for this purpose with amultiplier ll ofsuch high resistance as to eliminate substantially anyin- 30 ductive eflect of the voltmeter windings. How-' ever, since theapparatus responds to a critical speed and it is suflicient to detectthe appearance of current or voltage without measuring it, it is notnecessary to use a measuring instrument and a simple indicator or relay,or a relay of any suitable type such as shown in Fig.4 or in Fig. 6, e.g., may be used.

A running-light excitation curve for a predetermined speed .01 thesynchronous machine II is 40 shown at a in Fig. 2. In Fig. 2, voltageis'repre sented by distances measured vertically and current bydistances measured horizontally. The curve a accordingly representsgenerator voltage plotted againstexciting current and tends to taper oilin slope, owing to the eflect oi saturation. As is well known, aquadrature leading current in the load windings of an alternatingcurrentgenerator magnetizes the fleld and is the 4 equivalent of a magnetizingcurrent flowing in a special field winding. Accordingly, the curve a maybe plotted in terms oi quadrature leading current drawn from thegenerator windings, and load currents drawn by condensers connected tothe generator ll may be plotted on the scenes graph as the running-lightexcitation curve a.

The curves b and c represent the relationship between applied voltageand currents drawn by difierent condensers having two differentcapacities and are straight lines through the origin 0 owing to thelinear relationship between voltage and current in a condenser at anygiven frequency. The slopes of the curves 1? and c represent thecapacitative impedances of the condensers in question and it will beseen that the slope of the curve'b is so great as not to intersect thecurve a except at the origin, whereas the slope of the curve c'is suchthat the curve 0 intersects the curve a also at the point P.

Fig. 2 demonstrates graphically that if condensers having an impedancerepresented by the slope of the curve 0 are connected to the machine II,once excitation is started the excitation will be sustained and thevoltage and current will rise to the point P at which the curves 0. andc intersect and stability is attained. At the point P the ratio ofgenerator voltage to exciting or inducing current is equal to theimpedance of the condenser represented by the slope of the curve C.Ordinarily, there is sufficient initial excitation from residualmagnetization or other causes to start the rise in voltage and flow ofcurrent. However, if desired, a weak direct-current exciting field or aweak permanent magnet may be provided for initial excitation; or, ifdesired, rotor material may be employed having an especially highremanence. On the other hand, with a condenser impedance of suchma'gnitude as represented by the slope of the curve b, thecondenser-current curve and the generatorexcitation curve do notintersect, indicating that the generator cannot produce sumcient voltageto maintain current and the generator cannot excite itself.

It is apparent that, for a predetermined speed, there is a criticalvalue of capacity above which the machine becomes self-exciting.Conversely, for a predetermined value of capacitance and variable speed,there is a critical speed above which the machine becomes self-excitingsince generated voltage increases with speed whereas the condenservoltage for a given condenser current varies inversely with speed andfrequency.

'Fig. 2 if one considers that, with greater speeds,

the ordinates of the curve a will be raised and, since capacitativeimpedance varies inversely with generator speed- (owing to theproportionality between speed and frequency) the ordinates of thecondenser-current curves, such as curve b, will be lowered. Thischaracteristic of the system may be plotted as in Fig. 3, in whichvertical distances represent voltage and horizontal distances representspeed. Up toa predetermined speed, there is no appreciable excitationwhereas, above this speed, the machine becomesself-excited and thevoltage rises rapidly with speed.

The abrupt rise in voltage at a predetermined speed makes the apparatusof great value in speed-limiting apparatus for safety purposes. In placeof the voltmeter it, one may connectsuitable apparatus, such as anoverspeed alarm, a

device for cutting of! thepower and shutting down the apparatus, as byshutting oi! the fuel supply of aprime mover, or a device for correctingthe speed. For example, my speed-responsive device may be utilized as aturbine governor to regulate steam supply valves or as a motor speedregulator to vary the field strength, brush position, or some othercondition or a motor. Such speed-controlling or correcting apparatus maybe connected to the output windings of the generator, directly orindirectly, as will be explained more in detail hereinafter.

In the. arrangement of Fig. 6, for example, wherein a synchronousgenerator I I, such as that of Fig. l, is utilized to govern the speedof a turbine l8 or to govern the frequency of an alternator l9 driven bythe turbine l8, the windings of the generator H are connected directlyto the induction or commutator type valve motor 20. The valve motor 20serves to rotate a valve 2| through suitable gearing 22 in the directionof the arrow in order to reduce the fiow of steam supplied to theturbine l8 whenever the speed exceeds a predetermined value at which thegenerator ll becomes self-exciting. The motor 20.

continues to close the valve 2| further until the speed of the turbineI8 is reduced to the proper value. Suitable means, such as a springmotor 23, are provided for opening the valve 2| in case the turbinespeed fallsbelow the desired value.

Certain types of speed governors, such as flyball governors, in efiectmeasure centrifugal force and act when centrifugal force attains a predetermined value, which makes it difiicult to All struct such governorsfor very precise speed control. My governor has the advantage ofproducing a speed-correcting force abruptly at a predetermined speed sothat it acts positively to maintain such a speed. Another difficultyexperienced in mechanical governors is that the speed-responsivemechanism, which includes sliding parts, must overcome friction of somemoving part, and frequently also the force of a restraining spring inorder to act. Corrosion and otheruncontrollable circumstances changeboth friction and spring strength and introduce un-' certainty in theoperation of mechanical governors. Such difiiculties are overcome in mygovernor, however, since the speed-responsive element, itself, is whollyelectrical. My device, furthermore, affords great reliability andsafety, requires no independent current source, includes no electricalcontacts, and its operation is, therefore, not dependent upon thecontinuity of operation of such auxiliary apparatus.

Fig. 4 illustrates an embodiment of my invention in which the connectionbetween the speedresponsive generator and the speed-correcting apparatusis indirect instead of direct, in which an electric motor rather than aprimemover is to be held at a predetermined value. The motor 26 is;energized by a current source 21 and has a main shunt field 28 as wellas an auxiliary field 29. The design is such that the winding 29 may beemployed to eflect the entire variation in field strength that may berequired to hold the motor speed at a constant value under thevariations in operating conditions to which the motor may be subjected.

Mechanically connected to the motor 28 is an induction generator 30 witha squirrel cage rotor 3| and a stator winding 32. To provide excitingcurrent for-the generator 30, a condenser 33 is connected to the winding32. The speed-responsive generator is indirectly connected to the motorspeed controlling field winding 29 by means of a voltage or currenttransformer 34 with a rectifier 35 interposed between the motor fieldwinding 29 and the transformer 34.

In a manner similar to that explained in connection with the embodimentof Fig. 1, when a predetermined critical speed is attained, theinduction generator 38 will abruptly become exciting. As in the case ofthe embodiment of Fig. 1, residual magnetism may be relied upon forproducing initial excitation, or auxiliary means such as a smallpermanent magnet 3| carried by the rotor 3| may be employed. Thefrequency of the voltage and current produced will, as is well known tothose skilled in the art, be lower than the frequency of a synchronousmachine having a like number of poles and angular speed, as a result ofthe slip, characteristic of induction machines. Current will be inducedin the transformer 34 and rectified by the rectifier 35, causing anexciting current to flow in the motor field winding 29 and increasingthe field strength of the motor 26. Thereupon, the motor speed will falluntil the field 29 weakens and an equilibrium is reached.

In place of using the transformer 34 and rectifier 35 to bring aboutcorrection in the field strength of the motor 26, I may use any othersuitable appliance. For example, I may employ a carbon pile rheostat 36in series with the field and controlled by a solenoid winding 31connectible to the generator 30. To connect the apparatus for thismethod of operation, a double-pole, double-throw switch 38 is moved tothe upward position.

The supervision of the frequency of an alternating current network maybe accomplished by means of a speed-responsive generator driven byeither a synchronous motor or alternator or an induction motor connectedto the network in question. For example, as explained in connection withFig. 6, the generator H is responsive to the frequency of the network 24to which the alternator I9 is connected. However, if desired, a drivingmotor and a speed-responsive generator may be combined in one unit.

For example, as illustrated in Fig. 5, a doublewindinginduction machineis connected to the network 24, the frequency of which is to besupervised. The machine 48 is provided with stator windings forming twodifferent numbers of poles, the fields of which do not mutuallyinteract.

Although separate windings may be employed for the two sets of poles,some saving in material may be effected by employing a winding adaptedto be reco hected for two different numbers of poles, as lustrated inFig. 5.

The machine 48 employs a squirrel cage rotor 4| and a stator with twogroups of stator coils connected in Y. One group of coils 42, 48, 44, isconnected to a common Y point 45 and the other group of coils 48, 41,48, is connected to a second common Y point 48. The two Y groups areconnected in parallel to the three-phase network 24. A condenser 58 isconnected between the Y points 45 and 48 anda suitable voltage orcurrent-responsive device 5|, such as an indicator or relay, forexample, is also connected in the stator circuit. When the machine 48becomes selfexciting, a voltage is induced which, depending upon theconnection employed, may possibly be one and a half or six times thefrequency of the network 24.

For examplgin the arrangement represented by Figs. 5 and 7, the statorcoils form three parallel pairs for the three phases with respect to thenetwork 24. Assuming a two-pole machine for this connection, the coil 44is divided into two parts, 52 and 53 (Fig. 7), producing a flux in thedirection of the arrow 54, and the coil 48 is also divided into twoparts, 56 and 51, producing a flux in the direction of the arrow 55. Thefluxes 54 and 55 combine to form a resultant flux in which the poles areI88 mechanical degrees apart to produc e a two-pole machine.

In accordance with the well known theory of operation of poiyphasemachines, the windings of the other phases (not shown in Fig. 7) willproduce two-pole fluxes at successively later periods to combine withthe fluxes of the windings 44 and 48 and produce a two-pole rotatingfield.

With respect to the condenser 58, however, the coils 44 and 48 (Fig. 5)are in series and not in parallel and any current therein produces twoopposite fields, 58 and 59 (Fig. 7). In a similar manner, the coils 42and 46 produce two opposite fields 60 and GI, and likewise for the othercoils around the periphery of the stator. Since the polarity reversesevery thirty mechanical degrees, self-excitation produces twelve polesand tends to induce a current having six times the frequency of thenetwork 24. In brief, with respect to the network 24, the connection ofthe stator winding is three-phase two-pole but, with respect to thecondenser 50, the connection is single-phase twelve-pole.

The increased frequency of the condenser circuit has the advantage ofdiminishing the size of condenser required. In order to prevent thehigher frequency circuit from being short-circuited by the network 24,which is to be regulated, there are interposed inductances 82 in serieswith the stator coils and the network 24.

In an analogous manner, the voltage of a network may be kept constant byhaving a driving motor connected to the network, which motor varies inspeed with applied voltage as in the case of direct-current shuntmotors, and having a speed-responsive generator, such as described,rotated by the driving motor.

I have already referred to the fact that, if the frequency or speed isheld constant and the capacity is allowed to vary, there will be acritical value of capacity above which the machine becomesself-exciting. An abruptly rising curve,

similar to that of Fig. 3, may, therefore, be obtained, plotting voltageagainst capacity. This characteristic may be utilized for guardinghightension transmission lines or generators against grounds sincegrounds greatly influence the capacity. For example, a generatorsusceptible of self-excitation driven at a substantially uniform speedmay be connected to the conductors of a transmission line or to suchconductors and ground, so that the capacity of such a transmission lineconstitutes a leading-current load for the generator corresponding tothe condensers I8, 83 or 58. When the line becomes grounded, thecapacity thereof will change and affect the excitation of the generatorprovided it is so designed that the normal capacity of the transmissionline is near the critical capacity of the generator. Preferably thespeed of the generator is made such that frequency induced byself-excitation is different from the frequency of the power currentdelivered by the transmission line and the capacity-responsive generatordoes not constitute any appreciable load on the transmission line.

In carrying out my invention by employing the self-excitation ofdirect-current machines for supervising variable quantities, I am unableto use resistors which decrease in voltage-to-current ratio withfrequency as in the case of condensers.

However, I may obtain a relatively abrupt rise in generated voltage byemploying a loading resistor which falls off in resistance with appliedvoltage, such as carbon or a suitable composition of carbon andsilicon-carbide as described in United States Patent/No. 1,822,742,McEachron. Nevertheless, for the most precise work, I prefercommutatorless alternating-current machines to either direct-current orcommutator-type alternating-current machines, in order to eliminate theindefiniteness of brush contact resistance.

I have herein shown and particularly described certain embodiments of myinvention and cer tain methods of operation embraced therein for thepurpose of explaining its principle and showing its application but itwill be obvious to those skilled in the art that many modifications andvariations are possible, and I aim, therefore, to cover all suchmodifications and variations as fall within the scope of my inventionwhich is defined in the appended claims. 2

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A speed-responsive device comprising in combination, analternating-current generator susceptible of self-excitation with acondensive load and adapted to be driven at varying speeds in accordancewith variations in a speed to which the apparatus is intended torespond, a condenser connected to said generator, and a relay con-"nected in circuit with said generator, said condenser being of suchcapacity that its voltagecurrent curve intersects the running-lightexcitation curve of said generator plotted with respect to leadingcurrent drawn therefrom.

2. Apparatus responsive to a predetermined speed comprising analternating-current generator adapted to be self-excited by leadingcurrent drawn therefrom and adapted to be driven at varying speeds inaccordance with variations in a speed to which the apparatus is intendedto respond, a capacitative current-consuming device connected to saidgenerator, and a relay connected in a circuit with said generator, saidcurrent-consuming device having such a capacity that, at the frequencyof said generator corresponding to said predetermined speed, saidcurrent-consuming device has an impedance equaling the ratio of thevoltage of said generator to the leading current required to induce sucha voltage.

3. A speed-responsive device comprising in combination, adynamo-electric machine adapted to be self-excitedand adapted to bedriven at varying speeds in accordance with variations in a' speed towhich the apparatus is intended to respond, such that the ordinates ofits running-light excitation curve drawn with respect to leading currentdrawn therefrom rise with increasing speed, an impedance connected tosaid dynamoelectric machine having a voltage-current curve intersectingsaid excitation curve for a speed above a predetermined value, and arelay in circuit with said machine.

4. A speed-responsive device comprising in combination, adynamo-electric machine adapted to be self-excited, a current-consumingdevice connected to said machine and a relay in circuit with saidmachine, said current-consuming device having a variablevoltage-to-current ratio which decreases with increase-in machine speed.

5. A speed-responsive device comprising in combination, a stator havinga generating winding, a salient pole rotor, a condenser connected tosaid stator winding for drawing excitation therefrom, and a relay incircuit with said stator 5 winding, said rotor being adapted to bedriven at varying speeds in accordance with variations in a speed towhich the apparatus is intended to respond, the capacity of saidcondenser in relation to the electrical characteristics of said 10stator and rotor being such that said condenser draws an excitingcurrent from said stator winding only above a predetermined rotor speed.

6. A speed-regulating system for a direct-cur- '3 A frequency-responsivedevice for a polyphase network comprising in combination, analternating-current dynamo-electric having a double polyphase statorwinding with 30 the two polyphase parts connected to said network andacting conjointly with respect to said network to produce the samenumber of poles in both parts, and a condenser for producingselfexcitation interconnected between the parts of 35 said statorwinding.

machine 8. A frequency-responsive device for a polyphase networkcomprising in combination, an alternating-current dynamo-electric havinga plurality of phase windings exceeding 40 in number the number ofphases of said network, each winding being connected at one end to saidnetwork and at the other end to another of said windings, saidconnections being windings serving as intermediate points, and acondenser connected between intermediate points.

machine 9. A speed-responsive device comprising in combination, analternating-current dynamoelectrlc machine adapted to be self-excitedand adapted to be driven at varying speeds in accordance with variationsin a speed to which the apparatus is intended to respond, a condenserconnected to said machine and having sufiicient capacity to drawexciting current from the machine at frequencies above a valuecorresponding to a predetermined speed of the machine, and a relay incircuit with said machine.

10. A regulating system for a direct-current dynamo-electric devicecomprising in combina- 60 tion with said device, an alternatinf" current11. A speed regulating system for a driving device having a speedadjusting mechanism, said system comprising incombination with saiddriving device an alternating current generator driven by said device.said generator having an output winding, a condenser connected to saidwinding, a relay in circuit with said generator and connected inoperating relation to, said ad- Justing mechanism, said generator beingadaptedtobeseli-excitedbvleadingcurrentdrawn therefrom and saidcondenser being of such capacityastodrawlclirrtntto 10 the excitationcurrent oi said generator at a predetermined generator speed.

12. A regulating systemior varying acharacteristicoiarotatingeiectricalmachineinresponsetovariationsinspeacomprisinginoom-ERNST W.

